There have been significant technological advancements, especially in liver transplantation, for the treatment of patients afflicted with liver disease. However, these advancements have been hampered by the lack of a specific liver function diagnostic test that can provide accurate information on the degree, progression and the prognosis of hepatic disease. Our objective is to develop a quantitative liver function test which will be used to determine the degree and progression of hepatic dysfunction in end stage liver patients. The test will use a stable isotope (non- radioactive) labeled liver substrate, 13C-phenylalanine, for monitoring liver metabolism. The functional capacity and integrity of the liver will be determined by the amount of substrate metabolized to 13CO2 which is easily measured in the breath. It is our hypothesis that level of phenylalanine oxidation is a measure of hepatocyte function and will decrease in relation to the severity of liver dysfunction. The phase I results support this hypothesis. The noninvasive breath test will provide the physician specific and quantitative information not available in the current liver function battery. It will be simple to administer and inexpensive.